Sonic distance measuring devices (range finders) are well known; see e.g., U.S. Pat. No. 4,910,717, assigned to Sonin, Inc. and incorporated herein by reference.
Such measuring devices operate on the well-known sonar principal. They direct a sonic beam (a stream of acoustic waves) from the measuring device to the target and detect the time of transit from the measuring device to the target and back to determine the distance from the measuring device to the target. While sonar is typically used underwater, for instance for submarine purposes, sonic commercial type measuring devices are typically used in the building trades to measure relatively short distances, for instance, for determining floor area of a room of a house or generally to measure distances of five to about one hundred feet. Typically the sonic pulses are ultrasonic.
A number of such devices on the market, such as the Stanley Works Intellimeasure and devices supplied by Sonin, Inc., additionally include a laser pointer as part of the measuring device. The laser pointer is mounted co-axial with the sonic beam and projects a small visible dot on the object at which the distance measuring device is pointed. This provides an indication to the user where the device is pointed.
The present inventor, however, has determined that there is a drawback to this approach because such an ultrasonic measuring device actually emits a cone of acoustic waves. This acoustic cone has its vertex at the measuring device and its base at the target. This cone is relatively large in diameter at its base due to the nature of acoustic waves, so the acoustic cone at its base is much larger than the laser pointer dot. That is, any object within the base of the acoustic cone may reflect the acoustic waves and thus one may be measuring distance anywhere and everywhere within the acoustic cone. However, the user of such a device with the laser pointer feature may be easily misled into believing that he is measuring precisely the distance to the object reflecting the dot. Hence it is possible to obtain a measurement which is extremely inaccurate since the distance to the wrong object (or portion of the object) is being measured.